While it would be nice to see some trolls getting their just desserts, I don't think this is a good idea. SE excluded it for a reason.

If you don't like somebody's idea, post a comment about it. That's what I'm doing now =P

Since the system was introduced, I've seen idiotic posts with 0 or 1 thumbs up.. and GOOD posts with 10 or more. So it's easy to tell which ideas actually have merit. We don't need to hurt people's feelings by spamming them with dislikes.